This study examines the effect of adult attachment on post-traumatic growth among traumatized people and verifies the mediating role of self-differentiation. Thus, a research-based path model was developed to examine the mediating effect of self- differentiation in causal relationship between post-traumatic growth and adult attachment, including attachment anxiety and attachment avoidance. 500 adults aged 20 to 55 living in Korea who have had traumatic experiences were measured with a self-administered questionnaire, traumatic experience schedule, adult attachment scale, post-traumatic growth scale, and self-differentiation scale. Of those, 402 respondents were analyzed with SPSS 23.0 and AMOS 20.0 and conducted a structural equation modeling(SEM) analysis. Results indicated that first, attachment anxiety and attachment avoidance were negatively correlated with post-traumatic growth and differentiation of self, but differentiation of self and post-traumatic growth were positively correlated. Second, attachment anxiety and attachment avoidance had a significantly negative effect on post-traumatic growth and differentiation of self, but differentiation of self had a significantly positive effect on post-traumatic growth. Third, the differentiation of self had a mediating effect on the associations among attachment anxiety, attachment avoidance and post-traumatic growth. Adult attachment had an indirect effect on post-traumatic growth through differentiation of self and differentiation of self had a positive effect on post-traumatic growth. This study identifies that self-differentiation was significant in the process of adult attachment and post-traumatic growth of individuals while research regarding post-traumatic growth and self-differentiation were limited. With these results, the study provides information for counseling practices, its implications, and the limitations.